1. Field of the Invention
The present invention relates to drive coupling units for transmitting a torque via a fluid pressure produced dependent on a difference in rotational speed between two rotating shafts, and more particularly to a drive coupling unit suitable for use in a four-wheel drive unit for motor vehicles.
2. Description of the Prior Art
In a four-wheel drive vehicle in which front wheels and rear wheels are commonly driven by a single engine, it may occur that the effective turning radius differs slightly between the front wheels and the rear wheels, and the rolling path differs not only between the right and left wheels but also between the front and rear wheels when the vehicle is turning a corner, for instance. Since those differences occurring between the front and rear wheels are permitted by a differential unit, a full-time four-wheel drive vehicle is equipped with a center differential unit disposed between the front wheels and the rear wheels.
The center differential unit, however, cannot easily be constructed compactly and hence increases the weight and the manufacturing cost of a vehicle body. Furthermore, the center differential unit requires an additional component such as a differential lock mechanism for maintaining a four-wheel drivability. Such additional component enlarges the overall construction of the central differential unit.
With the foregoing difficulties in view, there have been proposed drive coupling units so constructed as to optionally transmit a torque from the front wheel side to the rear wheel side via a fluid pressure instead of the center differential unit.
The proposed drive coupling units comprise a drive rotating shaft disposed at a front wheel side connected with an engine, a driven rotating shaft disposed at a rear wheel side, and a working fluid acting on the drive and driven rotating shafts for producing a fluid pressure (or fluid resistance) dependent on a difference in rotational speed between the drive and driven rotating shafts, thereby optionally transmitting a torque to the rear wheel side.
FIG. 6 of the accompanying drawings diagrammatically shows the general construction of a vehicle driving system incorporating the conventional drive coupling unit stated above. The driving system includes a transversely installed engine 1 connected with a transmission 2 having an output shaft 2a on which is mounted a drive gear (or a fourth-speed counter gear) 2b coupled with a drive coupling unit 5.
The drive coupling unit 5 is mounted in a transmission case and includes, as shown in FIGS. 6 and 7, a cam ring 51 and a rotor 52 received in the cam ring 51, the cam ring 51 having on its outer periphery a cam ring gear 53 (FIG. 6). The can ring 51 is connected with the output shaft 2a through a meshing engagement between the cam ring gear 53 and the drive gear 2b.
The cam ring 51 is connected with a first rotating shaft 55 which is composed of a tubular outer shaft carrying thereon a gear 54. The gear 54 is held in mesh with a differential unit 6 so that the cam ring 51 is connected with front wheels 3, 3 through the gear 54 and the differential unit 6.
The rotor 52 is connected with an inner shaft (second rotating shaft) 56 extending through the tubular first rotating shaft 55 and connected via a bevel gear mechanism 7a to the front end of a propeller shaft 8. The rear end of the propeller shaft 8 is connected via a bevel gear mechanism 7b to a differential unit 9 to which axles of the respective rear wheels 4, 4 are connected.
With this construction, a driving force from the engine 1 is transmitted to the front wheels 3, 3 successively through the engine output shaft 2a, the cam ring 51, the first rotating shaft 55 and the gear 54 of the drive coupling unit 5, and the differential unit 6, while at the same time the engine driving force is optionally transmitted through the drive coupling unit 5 to the rear wheels 4, 4.
The drive coupling unit 5, as schematically shown in cross section in FIG. 7, includes the cam ring 51, the rotor 52 connected with the second rotating shaft 56 and rotatably disposed in the cam ring 51, and a multiplicity of radial sliding vanes 57 carried on an outer peripheral surface of the rotor 52 and held in sliding contact with an inner peripheral surface of the cam ring 51.
The vanes 57 are slidably received in corresponding radial slots 58 formed in the rotor 52 and hence they are movable in radial directions to project from and retract into the radial slots 58. Each of the radial slots 58 has an enlarged inner end portion 59 communicating with a pressure chamber 60.
The cam ring 51 and the rotor 52 define therebetween a plurality of pump chamters 61, 62, 63, each pump chamber 61, 62, 63 having at its opposite ends a pair of intake/discharge ports 61a, 61b; 62a, 62b; 63a, 63b. The pump chamber 61, 62, 63 is divided by the vanes 57 into a discharge side compartment and an intake side compartment. The pump chambers 61-63 are filled with a working oil.
The intake/discharge ports 61a, 62a, 63a are communicated with each other by a first oil passage (first working fluid flow passage) 64 while the intake/discharge ports 61b, 62b, 63b are communicated with each other by a second oil passage (second working fluid flow passage) 65.
The first and second oil passages 64, 65 are communicated together via an oil passage 66 in which an orifice 67 is disposed. The first and second oil passages 64, 65 are further communicated with an oil reservoir 70, respectively, through oil passages (working fluid supply passages) 68, 69 for supplying therethrough the working oil from the oil reservoir 70 to the pump chambers 61-63.
The first and second oil passages 64, 65 are further communicated with the pressure chamber 60 through oil passages 71, 72. Each of the oil passages 68, 69, 71, 72 has a check valve 73-76.
With this arrangement, when the first rotating shaft 55 and the second rotating shaft 56 creates a difference in rotational speed therebetween, the rotor 52 starts rotating relative to the cam ring 51.
For example, when the rotor 52 turns counterclockwise in FIG. 7 relative to the cam ring 51, the vanes 57 force or drive the working oil into the respective pump chambers 61-63 in which instance first sides of the individual pump chambers 61-63, in which the intake/discharge ports 61a-63a are disposed in front of the vanes 57, constitute discharge side compartments, whereas second sides of the individual pump chambers 61-63, in which the intake/discharge ports 61b-63b are disposed in the rear of the vanes 57, constitute intake side compartments.
A pumping action produced by the vanes 57 causes the working oil to be discharged from the intake/discharge ports 61a-63a, now acting as discharge ports, to the first oil passage 64 from which the working oil flows successively through the oil passage 65 and the second oil passage 65, then is drawn from the intake/discharge ports 61b-63b, now acting as intake ports, into the pump chambers 61-63, the direction of flow of the working oil being indicated by arrows in FIG. 7.
The working oil, as it flows through the orifice 67 in the oil passage 66, is subjected to a resistance acting in a direction to prevent the rotor 52 from rotating relative to the cam ring 51. The magnitude of the resistance is proportional to the amount of flow of the working oil.
Thus, the rotor 52 and the cam ring 51 are controlled by the action of the working oil in such a manner as to reduce the difference in rotational speed between the rotor 52 and the cam ring 51. For instance, when the cam ring 51 tends to rotate in excess relative to the rotor 52, a portion of the rotating torque is also transmitted to the rotor 52 via the working oil.
By the action of the drive coupling unit 5, the torque from the engine 1 can be transmitted to the front wheels 3, 3 and the rear wheels 4, 4 at such a proper distribution ratio that the front wheels 3, 3 and the rear wheels 4, 4 are driven to rotate substantially at the same speed. A four-wheel driving condition is thus achieved.
As a result, in the normal cruising condition where the slip of the front wheels 3, 3 is very small, the driving torque from the engine 1 is transmitted mainly to the front wheel side, while at the same time it occurs little or substantially no torque transmission to the rear wheel side.
On the other hand, the slip of the front wheels 3, 3 becomes large when the vehicle is running on a low friction surface such as a sandy land. In this instance, the torque from the engine 1 is transmitted to the front wheel side and the rear wheel side at a proper torque distribution ratio. With this torque distribution, the slip of the front wheels 3, 3 in reality is restricted to the least, thus ensuring that the vehicle while being driven by four wheels runs stably on such a low friction surface without causing undue slip of the front wheels.
The working oil discharged in the first oil passage 64 or the second oil passage 65 is pressurized and a portion of such pressurized working oil is then supplied through the oil passage 71 or the oil passage 72 to the pressure chamber 60. During that time, the check valve 75 or 76 permits the pressurized working oil to flow into the pressure chamber 60 while preventing the reverse flow of the pressurized working oil from the pressure chamber 60 toward the first oil passage 64 or the second oil passage 65. The pressure chamber 60 is therefore maintained at a pressure above a predetermined value with the result that the pressurized working oil acts on the inner ends of the respective sliding vanes 57, urging the sliding vanes 57 radially outwardly into pressure contact with the cam ring 51, thus providing an enhanced fluid-tightness.
The working oil may leak from a seal portion in the cam ring 51 or the rotor 52. When such leakage takes place, an adequate amount of working oil will be supplied from the oil reservoir 70.
In the drive coupling unit 5 of the foregoing construction, the difference in rotational speed between the first rotating shaft 55 and the second rotating shaft 56 is related to the transmitting torque (differential limiting torque) between the first and second rotating shafts 55, 56, as indicated by a solid line in FIG. 8. As appears clear from the same figure, the transmitting torque increases progressively with an increase in differential rotational speed. This drive coupling unit has a large torque transmitting (differential limiting) capability and generates only a small amount of heat as compared with another conventional unit used for full-time four-wheel driving, such as a viscous coupling. A full-time four-wheel drive vehicle having the aforesaid drive coupling unit has an improved on-demand four-wheel drivability which is a necessary capability of transmitting the engine torque to the rear wheel side on demand, and hence is capable of considerably reducing the possibility of occurring a tight braking phenomenon.
In the conventional drive coupling unit 5 stated above, the discharge side compartments in the respective pump chambers 61-63 and the intake side compartments in the respective pump chambers 61-63 are connected to the oil reservoir 70 respectively through the two oil passages 68, 69 each of which serves to provide a plurality (three being shown here) of discharge side compartments or intake side compartments with a supply of working oil. The oil passages 68, 69 are relatively long and hence have a relatively large flow resistance. For reliable supply of the working oil, it is necessary to increase or enlarge the cross-sectional area of the oil passages 68, 69. With this enlargement of cross-sectional area, it becomes necessary to provide large check valves 73, 74 in the thus-enlarged oil passages 68, 69. The large check valves 73, 74 are difficult to operate in immediate response to a change in the direction of flow of the working oil in the oil passages 68, 69 when the direction of rotation of the rotor 52 relative to the cam ring 51 is changed, thus lowering the overall performance of the drive coupling unit. Further, the drive coupling unit having such large check valves 73, 74 is also large in size as a whole.